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Abstract:Bulk permeability measurements provide an incomplete descriptor of permeability when considering heterogeneous or anisotropic membranes. Localized measurements of flux allow for independent assessment of constituent contributions to total permeability. Analogous to a puck on an air hockey table, permeate flow in membranes contributes to a local reduction in surface friction. By positioning a lateral force microscope (LFM) onto a custom membrane stage, local variations in friction force and hence permeability can be measured directly at the surface, with nanoscale resolution. In the absence of gas-membrane interactions, the friction force - pressure gradient is directly proportional to bulk permeability of the membrane. On a more complex reverse selective membrane, the friction force is shown to have both diffusive and sorption contributions.